Dispenser cathode for use in high power magnetron devices



Jan. 10, 1967 K. MACDONALD ETAL DISPENSER GATHODE FOR USE IN HIGH POWERMAGNETRON DEVICES Filed June 5, 1964 IFI,G.2

IN VENTORS. KENNETH A. MACDONALD and JOSEPH A SMITH EM 774 M4 AGENT.

United States Patent 3,297,901 DKSPENSER CATHQDE FOR USE IN HIGH PUWERMAGNETRON DEVICES Kenneth A. Macdonald and Joseph A. Smith,Williamsport, Pa, assignors, by mesne assignments, to Litton Industries,Inc, Beverly Hills, Calif, a corporation of Delaware Filed June 5, 1964,Ser. No. 372,804 16 Claims. (Cl. 313-346) This invention relates toemissive cathodes, particufearth compound with one or more refractorymetals.

The alkaline earth composition chosen is normally one which will beslowly reduced by the refractory metal at the operating temperature ofthe cathode to provide free alkaline earth metal at the emissive surfaceof the cathode, thereby enhancing the electron emitting capabilities ofthe surface and producing a copious supply of primary electrons.

As is well known in the operation of pulsed magnetrons, electrons fromthe=cathode which are not in proper phase to contribute energy to theoscillating mode of the magnetron return to the cathode. Since thereturning electrons strike the cathode with considerable energy,secondary electrons are expelled and the cathode is heated. Afteroscillation has been started, the cathode heater current is reduced andthe necessary energy for heating the cathode is largely obtained fromthe back bombardment by the electrons returning to the cathode. In highpower magnetrons, secondary electrons are the principal constituent ofthe cathode current; therefore, dispenser type cathodes for use in highpower magnetrons commonly employ materials which have good secondaryemission capabilities in addition to their primary electron emissioncapabilities. A frequently used exam ple of such a cathode is a poroustungsten body impregnated with barium calcium aluminate.

As mentioned above, the plentiful supply of primary electrons from adispenser type cathode depends on the availability of an adequatequantity of free alkaline earth metal at the emissive surface of thecathode. However, the quantity of free alkaline earth metal produced bycathodes most commonly used in magnetrons is more than sufficient forthis purpose, and some of the excess material evaporates from thesurface of the cathode and deposits on adjacent elements of themagnetron structure, such as the cathode and shields, the magnet polepieces, and the anode vane tips. The free alkaline earth metal causesspurious electron emission from these elements which interferes with theproper oscillating mode of the magnetron.

Various techniques have been employed in attempting to avoid theexcessive production of freealkaline earth metal. For example, theconventional impregnated tungsten cathode may be operated at a lowertemperature, or special cathodes may be fabricated employing materialswhich have good secondary emission capabilities but a low rate ofevaporation of alkaline earth metal, and consequently low primaryemission. However, high power magnetrons employing cathodes of thesetypes frequently have poor starting stability. That is, the mag netronmay not start to oscillate immediately upon the application of the fullpulsed voltage. The poor starting characteristics'of these devicesappear to be due to the fact that the cathode produces little or no freealkaline earth metal at the operating temperature, and therefore thecathode surface is susceptible to deactivation by poisoning.

It is an object of the present invention, therefore, to provide animproved cathode.

It is a more specific object of the invention to provide a dispensertype cathode which is capable of producing sufficient primary electronsto provide immediate starting of a high power magnetron but which doesnot evaporate an excessive quantity of emission enhancing material.

Briefly, in accordance with the foregoing objects, a cathode accordingto the invention comprises a major surface region of a material capableof emitting secondary electrons and a minor surface region of a materialwhich provides a source of primary electrons. The minor surface regionof the cathode may be constituted of a material which in itself is agood primary emitter of electrons or it may be of a reducing materialwhich reduces non-emitting compounds diffusing from the major surfaceregion to form a low work function material, thereby creating a regionof high primary emission at the interface between the reducing materialand the major surface region.

Additional objects, features, and advantages of cathodes according tothe invention will be apparent from the following detailed discussionand the accompanying drawings wherein:

FIG. 1 is a cross-sectional view in elevation of a portion of amagnetron having mounted therein a cathode according to one embodimentof the invention, and

FIG. 2 is a cross-sectional view illustrating a cathode according toanother embodiment of the invention.

A magnetron incorporating the first embodiment of the invention isillustrated in FIG. 1. The magnetron includes a cylindrical anode block10 having a plurality of vanes 11 extending radially inward therefrom toprovide a plurality of cavity resonators lying in a circular array aboutthe central axis of the anode. The anode is strapped with pairs ofconcentric metal rings 12 located in recesses in each end of the vanes.Each strapping ring is connected only to alternate vanes in the mannerwell known in the art in order to insure operation of the magnetron inthe 11' mode. Upper and lower magnet pole pieces 13 and 14 are sealed tothe anode block and are arranged to direct a magnetic field frompermanent magnets (not shown) through the magnetron interaction space.Electromagnetic energy is coupled from the device by an output coupler15 communicating with one of the cavity resonators.

An electron emissive cathode 20 according to the invention is positionedcentrally of the anode and spaced from the tips of the vanes. Thecathode is brazed to a cathode support 21 and passes through a bore 22in the lower pole piece 14. The cathode support 21, and pole piece 14are attached together by a suitable arrangement of insulating hermeticseals (not shown).

The cathode 20 includes a hollow cylindrical sleeve 25 of molybdenumwhich is sealed at its lower end to the cathode support 21 and whichextends centrally of the anode up into a recess 26 in the upper magnetpole piece. Non-emissive end shields 27 and 28 are staked to steps onthe molybdenum sleeve so as to confine the electrons and prevent theirmovement into the end spaces above and below the ends of the anodevanes. The end shields are separated from the electron emissive surface30 of the cathode by molybdenum spacers 31 and 32. A coiled heaterelement 33 of tungsten wound on a ceramic mandrel 34 is positionedcentrally of the molybdenum sleeve 25. One end of the heater element iselectrically connected to the cylindrical sleeve 25. The lower end ofthe heater element extends through a spacer in the bottom of the sleeveand passes externally of the magnetron through a suitable arrangement(not shown) to provide a terminal for supplying heater current to theheater element.

In the cathode according to the invention as illustrated in FIG. 1 theactive surface of the cathode includes a first or major surface regionhaving a material at the surface which is capable of emitting secondaryelectrons and has a low rate of evaporating of emission enhancingmaterial. Adjoining the ends of this region are two minor surfaceregions 41 and 42, each of which provides a source of primary electrons.The area of the minor surface regions is small relative to the area ofthe major surface region.

The major surface region 40 is formed of a cylinder of material which isa good secondary emitter of electrons but which has a 'low rate ofevaporation of free alkaline earth metal. The cylinder may be a matrixof refractory metal having dispersed therein a series of compounds of analkaline earth metal of the nature of, for example, tungstates andmolybdates which are not readily reduced to the free alkaline earthmetal by the refractory metal.

More specifically, the cylinder may be formed by compacting a mixture ofbarium carbonate and one or more refractory metals such as tungsten andmolybdenum and sintering at a high temperature in an inert atmosphere.Under such conditions tungstates and/or molybdates of barium are formed.Alternatively, the cylinder may be of porous tungsten, molybdenum, orrhenium impregnated with tungstates or molybdates of an alkaline earthmetal such as barium.

The minor surface regions 41 and 42 are formed of cylinders coaxial withthe cylinder forming the major surface region and adjoining each end ofthe major surface region. Each cylinder may be a matrix of refractorymetal having dispersed therein an alkaline earth metal compositioncapable of furnishing alkaline earth metal to the surface of thecylinder. More specifically, the cylinders may be annular members orwashers of porous tungsten impregnated with a barimum calcium aluminateof relatively low barium content.

Alternatively, the minor surface regions 41 and 42 may be formed of amaterial which is of strongly reducing nature such that non-emittingcompounds diffusing from the adjoining secondary emitting region arereduced and a low work function material is formed, thereby creating aregion of high primary emission at the interface between the major andminor surface regions. More specificially, rings or washers of tantalum,zirconium, titanium. or niobium may be employed, tantalum beingpreferred.

The total surface area of the minor surface regions 41 and 42 desirablyshould be of the order of 20% or less of the total emission surface areaof the cathode. Specifically, cathodes have been fabricated in which themajor surface region 40 was provided by a cylinder .100 inch high of acompacted mixture of 10% by weight barium carbonate, 45% by weighttungsten, and 45% by weight molybdenum, sintered at 1700 C. in pure drynitrogen. This cylinder was employed with two washers 41 and 42 each.010 inch thick of porous tungsten impregnated with barium calciumaluminate of the composition BaO-2CaO-2Al O Alternatively, tantalumwashers .010 inch thick have been employed in place of the impregnatedtungsten washers.

FIG. 2. illustrates a second embodiment of a cathode according to theinvention which may be mounted in the magnetron of FIG. 1. In thisembodiment the cylindrical surface of secondary electron emittingmaterial is provided by two cylinders 51 and 52. Intermediate the twocylinders and adjoining an end of each is a narrow cylindrical region 53of material providing a source of primary electrons. The materialsemployed for producing primary and secondary electron emission may bethe same materials as those employed in the first embodiment of theinvention.

Cathodes according to the invention have been utilized in high powermagnetrons with excellent results. Starting stability has beensatisfactory indicating that sufficient free alkaline earth metal isavailable at the surface of the minor surface regions to keep thesurface active and not susceptible to deactivation by poisoning.Interference with the proper operating mode of the device has beeneliminated indicating that there is no significant excess of freealkaline earth metal available to establish sources of spuriouselectrons on elements of the device adjacent the cathode.

What is claimed is:

1. A cathode comprising a first surface region of material capable ofemitting secondary electrons and having a low rate of evaporation ofemission enhancing material, and

a second surface region of material providing a source of primaryelectrons.

2. A cathode comprising a first surface region of material having a highsecondary electron emission yield and a low rate of evaporation of freealkaline earth metal, and

a second surface region of a refractory metal having dispersed thereinan alkaline earth compound capable of furnishing primary electronemission at the surface adjoining the first surface region,

the surface area of the second surface region being small relative tothe surface area of the first surface region.

3. A cathode according to claim 2 in which said first surface regioncomprises a matrix of refractory metal having dispersed thereincompounds of an alkaline earth metal which are not readily reduced tofree alkaline earth metal by the refractory metal of the matrix.

4. A cathode according to claim 2 in which said first surface regioncomprises a matrix of refractory metal selected from the groupconsisting of tungsten, molybdenum, and rhenium having dispersed thereina material selected from the group consisting of tungstates andmolybdates of an alkaline earth metal.

5. A cathode according to claim 2 in which said first surface regioncomprises a material produced by compacting and sintering a mixture ofan alkaline earth metal carbonate and a refractory metal selected fromthe group consisting of tungsten and molybdenum.

6. A cathode comprising a first surface region of material having a highsecondary electron emission yield and a low rate of evaporation of freealkaline earth metal, and

a second surface region of material capable of reduc ing non-emittingcompounds diffusing from the first surface region to create a region ofprimary electron emission at the interface of the first and secondsurface regions.

the surface area of the second surface region being small relative tothe surface area of the first surface region.

7. A cathode according to claim 6 in which said first surface regioncomprises a matrix of refractory metal having dispersed thereincompounds of an alkaline earth metal which are not readily reduced tofree alkaline earth metal by the refractory metal of the matrix.

8. A cathode according to claim 6 in which said first surface regioncomprises a matrix of refractory metal selected from the groupconsisting of tungsten, molybdenum, and rhenium having dispersed thereina material selected from the group consisting of tungstates andmolybdates of an alkaline earth metal.

9. A cathode according to claim 6 in which of both the first and secondmaterials.

12. A cathode according to claim 11 in which said first materialcomprises a material produced by compacting and sintering a mixtureincluding by weight about barium carbonate, 45% tungsten, 40

said second material comprises a porous tungsten body impregnated withbarium calcium aluminate of com position BaO ZCaO- 2Al O 14. acylindrical cathode comprising a first material comprising a matrix ofrefractory metal said first surface region comprises a material produced5 having dispersed therein compounds of an alkaline by compacting and'sintering a mixture of an alkaearth metal which are not readily reducedto the free line earth metal carbonate and a refractory metal alkalineearth metal by the refractory metal, selected from the group consistingof tungsten and a second material capable of red i g non-emittingmolybdenum, 10 compounds diffusing from the first material to create 10.A cathode according to claim 6 in which said a region of primaryelectron emission at interfaces second surface region comprises arefractory metal sebetween the first and second materials, lected fromthe group consisting of tantalum, niobium, a fi st cy ndrical bodyconstituted of one of said mazirconium and titanium. terials,

11. A cylindrical cathode comprising a second cylindrical bodyconstituted of the other of a first material comprising a matrix ofrefractory metal id m e ia s coa al With the first cylindrical bodyhaving dispersed therein compounds of an alkaline and adjoining firstylin i al y t ne nd earth metal which are not readily reduced to thefree thereof, alkaline earth metal by the refractory metal, a thirdcylindrical body constituted of the other Of a second materialcomprising a matrix of refractory aid ma erials oaxial With the firstand second metal having dispersed therein an alkaline earth cylindricalbodies and adjoining the first cylindrical metal composition capable offurnishing primary body at the other end thereof, electron emission atthe surface, the cylindrical surface area of the second material a firstcylindrical body constituted of one of said mabeing less than 20% of thecylindrical surface area terials, of both the first and secondmaterials. a second cylindrical body constituted of the other of 15. Acathode according to claim 14 in which said materials coaxial with thefirst cylindrical body id fi t i l cemprises a i l produced by andadjoining the first Cylindrical y at 0116 end compacting and sintering amixture including by thereof, and weight about 10% barium carbonate, 45tungsten, a-th rd cylindrical body constituted of the other of andmolybdenum, and

mammals ,Coaxlal the first and said second material comprises a washerof tantalum. cylindrical bodies and adJommg the first cylindrical ACathode according to claim 14 in which body.at h other end thereof saidfirst material comprises a porous tungsten matrix the cylindricalsurface area of the second material im re mated with mnostates of bariumbeing less than 20% of the cylindrical surface area p g said secondmaterial comprises a washer of tantalum.

References Cited by the Examiner UNITED STATES PATENTS and molybdenum,and 2,698,913 1/1955 Espersen 313-446 said second material comprises aporous tungsten body 2,957,100 10/1960 ESPCFSBII et a1 313346impregnated with barium calcium aluminate of com- 3,027,489 3/1962Tulnlla 313346 X position BaO 2Ca0 2Al O 13. A cathode according toclaim 11 in which said first material comprises a porous tungsten matriximpregnated with tungstates of barium,

45 JOHN W. HUCKERT, Primary Examiner.

A J. JAMES, Assistant Examiner.

1. A CATHODE COMPRISING A FIRST SURFACE REGION OF MATERIAL CAPABLE OFEMITTING SECONDARY ELECTRONS AND HAVING A LOW RATE OF EVAPORATION OFEMISSION ENHANCING MATERIAL, AND A SECOND SURFACE REGION OF MATERIALPROVIDING A SOURCE OF PRIMARY ELECTRONS.